How do I adjust the flush duration on a dual-flush toilet?

You can't actually alter the flush duration that's governed by the amount of water in the cistern and whether you use full or half flush (twin buttons).
Check your cistern water level it might not be full enough for a decent flush. Adjust the water inlet valve float adjusting rod (depends on type) if the water level in the cistern is lower than the water level mark on the inside of the cistern wall, a mark about 3cm 1.5inch long. The other point is it may be the syphon diaphragm is damaged and needs replacing search net for how to replace a syphon diaphragm. You could also check the button plung rods under the buttons mine seem to screw themselves down a bit in use indicated by the buttons being lower than the surrounding ring. draw them up so buttons sit level wit side ring(button housing). again this depends on type.
Hope this helps

Bob UK
December 2013